In recent years there has been a trend towards low-phosphate and phosphate-free detergent formulations. To this end a number of non-phosphate detergent builders and water softeners have been developed. Na zeolite A, a synthetic aluminosilicate of composition NaAlSiO.sub.4, has been used in high volumes for many years and is as effective as sodium tripolyphosphate (STPP) at removing calcium but not as effective at removing magnesium. This aluminosilicate zeolite has recently been joined by zeolite P (European Patent Applications 0 384 070 and 0 565 364) as a commercially used builder which shows enhanced exchange kinetics. Alternative technologies are based on soluble silicates (amorphous and crystalline) which soften water effectively and generally show better magnesium removal than Na zeolite A. A crystalline layered sodium silicate SKS-6 (Na.sub.2 Si.sub.2 O.sub.5), which is used commercially, has also been developed by Hoechst AG and is described in U.S. Pat. Nos. 4,664,839, 4,820,439, 4,950,310 and 5,308,596. Also, crystalline sodium silicates with the kanemite structure and composition NaHSi.sub.2 O.sub.5.xH.sub.2 O have recently been developed by Hoechst AG, as described in European Patent Application 0 627 383.
Synthetic alkali magnesiosilicates having an anhydrous composition of xM.sub.2 O.ySiO.sub.2.zM'O where M represents Na and/or K; M' represents Ca and/or Mg; y/x is 1.4 to 2.1; z/x is 0.001 to 1.0; K/Na in M.sub.2 O is 0 to 80; and Mg/Ca in M'O is 0 to 100 have recently been proposed by Kao Corporation in European Patent Application 0 630 855. These materials, which have a chain silicate structure as described in European Patent Application 0 550 048, are shown to have high calcium binding capacity and to have utility as water softeners and as alkali adjusting agents. In addition, they are described as particularly useful for their good moisture resistance (Japanese Patent Application Kokai 07,330,325).
Synthetic alkali magnesiosilicate compounds with the general formula M.sub.2-2x Mg.sub.1-x Si.sub.1+x O.sub.4, where M is an alkali metal, have been reported previously, as discussed below. However, these highly crystalline compounds have not been recognised as having properties that enable them to be used as water softeners or detergent builders.
The compounds Na.sub.2 MgSiO.sub.4 (R. D. Shannon, Phys. Chem. Miner. 4, 139-148, 1979), Na.sub.4 Mg.sub.2 Si.sub.3 O.sub.10 (C. M. Foris et al. J. Appl. Cryst. 12, 405-406, 1979) and K.sub.2 MgSiO.sub.4 (E. W. Roedder, Am. J. Sci., 249, 224248, 1951; A. S. Berezhnoi et al. Izvestiya Akademiii Nauk SSSR, Neorganicheskie Materialy 12, 1653-1658, 1976) have all been described as having structures closely related to that of the silica polymorph, cristobalite (see FIG. 1). It has also been proposed (E. W. Roedder, Am. J. Sci., 249, 224-248, 1951) that in terms of the general formula M.sub.2-2x Mg.sub.1-x Si.sub.1+x O.sub.4, when M=K and x=0.5, i.e. KMg.sub.0.5 Si.sub.1.5 O.sub.4, a compound is formed which has a structure closely related to that of the silica polymorph, tridymite (see FIG. 2).
Tridymite and cristobalite both have the composition SiO.sub.2 and comprise a 3-dimensional framework of corner-connected SiO.sub.4 tetrahedra. They are classified as framework silicates or tectosilicates.
By analogy with alkali aluminosilicate analogues, the cristobalite- and tridymite-related compounds described above can also be described as stuffed derivatives of the cristobalite and tridymite structures (M. J. Buerger, American Mineralogist 39 600-614, 1954), and therefore as stuffed silica polymorph-related structures, in that up to half of the silicon cations in the silicate framework in each case are replaced by magnesium cations. Alkali cations, which are required for charge balance (Si.sup.4+ &lt;--&gt;Mg.sup.2+ +2M.sup.+, M=alkali) occupy the interstices in the respective frameworks (see FIGS. 1 and 2)--hence the descriptions "stuffed cristobalite" and "stuffed tridymite". Other stuffed silica polymorph-related structures include "stuffed quartz".
All of the above alkali magnesiosilicate compounds having stuffed silica polymorph-related structures were prepared from synthetic reagents and under reaction conditions that promoted the formation of very well crystallised and ordered materials.